1. Field of the Invention
The present invention generally relates to a heat sealer of a type capable of heat-sealing a sheet of a synthetic resin to form a bag and, more particularly, to horizontal and vertical sealing jaw assemblies for accomplishing horizontal and vertical seals, respectively.
2. Description of the Prior Art
The prior art weighing and packaging system will first be discussed with reference to FIG. 10 since the heat sealer to which the present invention pertains is generally employed in such weighing and packaging system. As shown in FIG. 10, the weighing and packaging system comprises a weighing apparatus 1 and a packaging machine 2 for successively packaging articles M, that are successively discharged from the weighing apparatus 1, with a sheet of a synthetic resin by forming such resinous sheet into bags one for each article M. Each article M may be one or a quantity of edible or non-edible items, for example, potato chips, fruits, candies, vegetables, screws, nails, bolts or other things desired or required to be bagged.
The weighing apparatus 1 is of a design operable to perform a combinational weighing by weighing the article M and then discharging the article M of a predetermined weight onto the packaging machine 2. This weighing apparatus 1 comprises a dispensing feeder 4 of a generally inverted conical shape for receiving articles M from a hopper 3 positioned thereabove and for dispensing the articles M towards an outer peripheral portion thereof, a plurality of vibratory feeders 5 disposed below the outer peripheral portion of the dispensing feeder 4 for receiving the respective articles M dispensed from the dispensing feeder 4 and subsequently transferring the articles M to a corresponding number of pool hoppers 6 positioned below the associated vibratory feeders 5, weighing hoppers 7 positioned below the corresponding pool hoppers 6 for receiving the respective articles M and operatively coupled with corresponding weighing cells 8 for measuring respective weights of the articles M within the weighing hoppers 7.
A control device 9 is of a design operable to select a combination of some or all of the weighing hoppers 7 which have measured the respective weight of the article M that falls within a predetermined weight tolerance, to cause respective gates of some or all of the weighing hoppers 7 so selected to open to allow the articles M to be discharged onto a collective discharge chute 10, and then to open a timing hopper 11, which is used to close a bottom opening of the collective discharge chute 10, so that the article M can be discharged onto the packaging machine 2 at a predetermined timing.
The packaging machine 2 comprises a bag former 12 positioned below the tiring hopper 11 of the weighing apparatus 1 for folding the synthetic sheet S inwardly along a longitudinal center line thereof so as to bring opposite longitudinal side portions of the strip of synthetic sheet to overlap with each other, a pair of juxtaposed endless pull-down belts 13 and 13 positioned below the bag former 12 for pulling the folded sheet S downwards, a vertical heat-sealing means (not shown) for heat-sealing opposite longitudinal side edges of the folded sheet S together to thereby shape the folded sheet S into a tubular form, and horizontal heat-sealing means 14 positioned below the vertical heat-sealing means for forming a transverse seal in the tubular sheet S to complete a bag.
The horizontal heat-sealing means 14 referred to above includes a pair of transverse sealing jaws 15 and 15 spaced a distance from each other and positioned on respective sides of the path of travel of the tubular sheet S. The transverse sealing jaws 15 and 15 are drivingly connected with respective drive means 16 and 16 so that the transverse sealing jaws 15 and 15 can be turned in respective senses opposite to each other so as to depict a generally D-shaped orbit as shown by the arrow in FIG. 10. More specifically, the drive means 16 and 16 are so drivingly synchronized with each other and also with the weighing apparatus 1 that starting from a top point PI at which the transverse sealing jaws 15 and 15 cooperate with each other to clamp the tubular sheet S, the transverse sealing jaws 15 and 15 can move downwards towards a bottom point P2 while pressing the tubular sheet S, then separate at the bottom point P2 away from each other and also from the tubular sheet S and finally swing backwards towards the top point P1. That portion of the tubular sheet S pressed by the transverse sealing jaws 15 and 15 is, during the downward movement of the transverse heat-sealing jaws 15 and 15 from the top point P1 to the bottom point P2, heat-sealed. Reference numeral 17 represents a control device for controlling the drive means 16 in operative association with the operation of the weighing apparatus 1. This type of the transverse heat-sealing means 14 is well known to those skilled in the art and is disclosed in, for example, the published International Application WO93/07058 and also in the Japanese Laid-open Patent Publication No. 62-235006.
Each of the transverse heat-sealing jaws 15 has such a cross-sectional shape as shown in FIG. 11A and includes a generally U-shaped support block 18 made of, for example, stainless steel and adapted to be carried by the associated drive means 16 (FIG. 10) forming a part of a press mechanism, and upper and lower presser pieces 20a and 20b made of, for example, stainless steel and having respective upper and lower sheet contact faces 19a and 19b. The support block 18 is of one-piece construction including upper and lower connecting arms 21 positioned on respective upper and lower sides of the U-shaped cavity in the support block 18, and the upper and lower presser pieces 20a and 20b are received within the U-shaped cavity in the support block 18 and are firmly connected to the respective upper and lower connecting arms 21 by means of set screws with the sheet contact faces 19a and 19b oriented in a direction away from the support block 18. The upper and lower presser pieces 20a and 20b rigidly connected to the support block 18 in the manner described above are spaced a distance from each other to define a cutter groove 22 in which a cutter blade 30 for severing the tubular sheet S is accommodated.
The support block 18 is formed with upper and lower cylindrical heater chambers 23a and 23b in which respective upper and lower rod heaters 24a and 24b are snugly fitted. Each of the presser pieces 20a and 20b has a respective heat pipe hole 25a or 25b formed therein at a location adjacent the corresponding sheet contact face 19a or 19b so as to extend a full width (as measured in a direction perpendicular to the sheet of the drawing of FIG. 11A or widthwise of the tubular sheet S) of the respective presser piece 20a or 20b, in which hole 25a or 25b is accommodated a corresponding heat pipe 26a or 26b. A temperature sensor 28 comprising a thermocouple encased in an enclosure is accommodated within a single sensor hole 27 formed in the support block 18 at a location between the lower heater hole 23b in the support block 18 and the heat pipe hole 25b in the lower presser piece 20b and beneath the imaginary line connecting therebetween. It is to be noted that in the following description, characters "a" and "b" affixed to reference numerals used to denote respective upper and lower portions of each component part may not be used.
FIG. 11B illustrates the pattern of distribution of temperature in one of the transverse heat-sealing jaws 15 measured at a portion thereof generally intermediate of the lengthwise direction thereof. It is to be noted that, in order for the illustrated transverse heat-sealing jaw 15 to be comparable with a counterpart (one of transverse heat-sealing jaws) embodying the present invention as will be described later, the transverse heat-sealing jaw 15 shown in FIG. 11B is assumed to have been formed of one-piece construction using the same material in the same cross-sectional shape as those of the counterpart of the present invention with the holes 23, 25 and 27 defined at the same respective positions as those in the counterpart of the present invention.
According to the pattern of temperature distribution shown in FIG. 11B, it is clear that a temperature difference of 17 to 18.degree. C. is found between a portion of the transverse heat-sealing jaw 15 adjacent the sensor hole 27 and any of the sheet contact faces 19a and 19b and that depending on the condition under which heat is radiated a temperature difference is developed between the sheet contact faces 19a and 19b, with the temperature of the upper sheet contact face 19a being higher than that of the lower sheet contact face 19b, along with a varying distribution of a lower temperature range. Any of the upper and lower sheet contact face 19a and 19b is preferred to be heated to a uniform temperature of, for example, about 150.degree. C. However, if the temperature to which any of the upper and lower sheet contact faces 19a and 19b is heated is lower than, for example, 140.degree. C., the tubular sheet S will be sealed insufficiently to such an extent as that opposed portions of the tubular sheet S, which have been pressed by the transverse heat-sealing jaws 15 and 15, will be separated easily, but if such temperature is too high, the tubular sheet S will be fused or heat-shrunk locally accompanied by reduction in sealing strength.
Considering that the tubular sheet formed by heat-sealing the opposite longitudinal side edges of the folded sheet S by the action of the vertical heat-sealing means as hereinbefore discussed has a longitudinal sealed portion in the form of, for example, three- or four-fold hem that extends longitudinally of the tubular sheet S and that at the sealing station where the transverse heat-sealing jaws 15 and 15 form a transverse seal in the tubular sheet S the multi-fold hem, that is, the longitudinal sealed portion in the tubular sheet S is aligned with respective portions of the upper and lower sheet contact faces 19a and 19b substantially intermediate of the width thereof, each of the sheet contact faces 19a and 19b of the upper and lower presser pieces 20a and 20b then heated will exhibit such a pattern of temperature distribution that a substantial amount of heat is absorbed at that intermediate portion of the respective sheet contact face 19a or 19b by the tubular sheet S with the temperature thereof consequently lowered as compared with the remaining portion of the respective sheet contact face 19a or 19b. As a result, the resultant transverse seal in the tubular sheet S will exhibit a lower sealing strength at a portion thereof registered with and adjacent the longitudinal sealed portion of the tubular sheet C than at the remaining portion thereof.
In the prior art transverse heat-sealing jaws discussed hereinbefore, although the heat pipes 26a and 26b utilized to equalize the temperature distribution in the respective sheet contact faces 19a and 19b are effective to accomplish this objective as long as they are in operation over a substantial length of time, the prior art transverse heat-sealing jaws 15 and 15 have no capability of quickly compensating for reduction in temperature at those intermediate portions of the respective sheet contact faces 19a and 19b immediately following the transverse sealing action performed thereby. This lack of the capability of quickly compensating for reduction in temperature has long led to reduction in sealing strength in that portion of the transverse seal which is aligned with the longitudinal sealed portion in the tubular sheet S.
Also, since the temperature sensor 28 is positioned offset from the path of direct transmission of heat from the heaters to the adjacent sheet contact faces in the respective presser pieces, the actual temperature at the sheet contact faces considerably differs from the temperature measured by the temperature sensor, making it difficult to control the temperature of the sheet contact faces accurately and with high precision.
Furthermore, the prior art transverse heat-sealing jaws have another problem associated with the use of the single temperature sensor. Specifically, the use of the single temperature sensor in each of the transverse heat-sealing jaws is unable to detect the difference in temperature between the upper and lower sheet contact faces in each transverse heat-sealing jaw and, there is no way of eliminating the temperature difference between the sheet contact faces in each transverse heat-sealing jaw.